The present invention relates generally to a radial flow reactor design with integrated thermal energy exchange, and to methods for using this apparatus to carry out catalyst-facilitated hydrocarbon processing.
A variety of familiar hydrocarbon processing operations are facilitated by contacting the hydrocarbon with a selected catalyst in a fixed or fluidized bed under controlled temperature and pressure conditions. One such conventional hydrocarbon processing operation involves the catalytic dehydrogenation of an alkylaromatic hydrocarbon in the presence of steam to a corresponding alkenylaromatic hydrocarbon, for example dehydrogenating ethylbenzene to produce styrene as taught by U.S. Pat. No. 5,461,179 (Chen et al.), which patent is incorporated herein by reference.
The efficiency of these catalyzed hydrocarbon processing operations can often be improved by utilizing a sequence of two or more catalyst beds in line. Thus, the effluent from a first catalyst bed or reaction zone, containing predominantly the desired final product together with unreacted hydrocarbon, is fed to a second catalyst bed or reaction zone located downstream from the first, where further reaction takes place to further increase the concentration of the desired product in the effluent from the second catalyst bed or reaction zone. In the same fashion, a third, fourth, or additional in-line, downstream catalyst beds/reaction zones may be added as desirable.
Between the two or more catalyst beds/reaction zones, the effluent from an upstream catalyst bed/reaction zone may have to be heated or cooled (depending on whether the reaction is endothermic or exothermic) to properly prepare it for further conversion to the desired product in the next downstream catalyst bed/reaction zone. Thus, if the catalyzed reaction is essentially endothermic in nature, the effluent will have to be heated between two catalyst beds/reaction zones to insure that the downstream catalytic conversion proceeds efficiently, or possibly, at all.
One such endothermic reaction is the catalyzed dehydrogenation of ethylbenzene to styrene. Thus, as described in U.S. Pat. No. 5,461,179, located in-line between the upstream catalytic reactor 50 and the downstream catalytic reactor 54 of that patent is an external reheater 52 to reheat the effluent coming from upstream reactor 50. Typically in such ethylbenzene dehydrogenation, the endothermic reaction is carried out in two or more single bed adiabatic reactors, with effluent from an upstream reactor being reheated in an external shell and tube exchanger before being fed to the downstream reactor. Performing the reheat step in this manner results in additional pressure drop (due to high frictional losses in the exchanger tubes), as well as an increase in void volume (empty space) because of the additional piping required. Higher system pressure results in yield losses to low value byproducts, and lowers apparent catalyst activity (due to equilibrium and coking effects). Larger void volume results in yield losses and formation of undesirable product impurities via non-selective thermal reactions. Therefore, it is highly advantageous to devise an economical way of circumventing these limitations of the conventional process.
The utility of multi-stage catalytic reactor designs is therefore limited by a variety of physical, economic, process, and thermodynamic factors. Because of space considerations, more compact reactor designs are generally desirable. Some catalyzed hydrocarbon reactions, such as ethylbenzene-to-styrene, benefit from maintaining relatively low operating pressures. The ability to rapidly add relatively large amounts of heat to the effluent between the reactor stages of the ethylbenzene-to-styrene process is limited by economic, metallurgical, and thermodynamic considerations. Thus, if superheated steam is used to reheat the effluent it may be necessary to use steam at extremely high temperatures to provide sufficient thermal energy in the limited mass of added steam. That in turn may require the use of more expensive, thermally-resistant materials in connection with the reheater. Accordingly, it is desirable to develop an improved design for a multi-stage catalytic reaction process that would alleviate some of the problems inherent in the prior art reactor designs.
Various types of so-called radial or axial/radial flow reactor designs are known in the art for various applications whereby at least a part of a process stream moves, at some point, through the reactor in a radial (i.e., inward-to-out or outward-to-in) direction, as opposed to the more familiar axial flow (i.e., end-to-end) reactor designs. For example, U.S. Pat. No. 4,321,234, which is incorporated herein by reference, discloses a type of radial flow reactor involving a single reaction chamber. This apparatus comprises an intercylinder chamber defined by a gas-permeable, cylindrical outer catalyst retainer, which is disposed inside an outer shell, and a gaspermeable, cylindrical inner catalyst retainer provided within the outer catalyst retainer. A plurality of vertically.extending heat-exchanging tubes are arranged in the reaction chamber in circular groups which are concentric with the common central axis of both of the catalyst retainers. A feed gas is supplied to either the outer gas flow passage or the inner gas flow passage and is caused simultaneously and uniformly to flow in all radial directions, either radially outwardly or radially inwardly. That is, the gas makes one pass through the entire annular extent of the cross section of the catalyst bed.
Another earlier patent, U.S. Pat. No. 4,594,227, which is incorporated herein by reference, discloses a reactor in which a feed gas is caused to flow radially through a catalyst bed packed in an annular space defined by two coaxial cylinders having different diameters. A vertically extending, annular, inter-cylinder space, defined between an outer catalyst retainer cylinder and an inner catalyst retainer cylinder, is divided into a plurality of chambers by radially extending vertical partition walls. Heat exchanging tubes are disposed vertically in the chambers for maintaining the proper temperature for the catalytic reaction. A catalyst is packed in the chambers, forming reaction chambers through which a feed gas flows-in radial directions. The heat exchangers make it apparent that this reactor is indirectly fired and depends on convective heat transfer.
U.S. Pat. No. 4,909,808, which is incorporated herein by reference, improves on the reactor design of U.S. Pat. No. 4,594,227 by providing a steam reformer contained within a cylindrical structure having a catalytic reactor tube of annular shape. Rather than using an external heating device to bring hot gases into the reactor tube, this invention utilizes a type of catalytic combustor located at the center of the cylindrical structure. Thus, two different catalytic reactions are taking place: one reaction common to catalytic reaction tubes of steam reformers, and a second reaction for creating the heat required for the steam reformer. This internal placement of the heat source and use of a catalytic combustor enhances heat transfer by both radiation and convection. The improvement in these characteristics is primarily due to the ability to control the heat flux (the amount of heat available from the fuel on the outside of the reactor tube) so as to match the amount of heat required by the reaction taking place inside the catalyst bed with the heat and temperature of the combustion gas outside the reactor.
Another so-called radial flow catalytic reactor is shown in U.S. Pat. No. 4,714,592, which is incorporated herein by reference. In this case because the targeted catalytic reaction is exothermic, there is a need to remove excess heat from the reaction environment. This is achieved by means of inlet and outlet pipes containing a coolant which is circulated through a coolant passage structure that penetrates the catalyst bed in order to absorb the heat of reaction. Other patents showing at least partial radial flow reactor designs include U.S. Pat. Nos. 4,230,669; 5,250,270; and 5,585,074, each of which is also incorporated herein by reference.
None of the foregoing patents, however, show a reactor design that is truly well suited for efficient single or multi-stage radial reactor processing of a hydrocarbon wherein the catalytic reaction is highly endothermic or exothermic in nature, thereby requiring respectively either significant and highly uniform heat inputs to the process stream or heat removal from the process stream before and/or after a single catalyst bed or before, after, and/or between serial catalyst beds. These and other drawbacks with and limitations of the prior art reactors are overcome in whole or in part with the reactor design of this invention.
Accordingly, a principal object of this invention is to provide a means of integrated thermal energy exchange in a radial flow reactor design for single or multi-stage catalytic bed processing of a hydrocarbon.
It is a general object of this invention to provide a compact, efficient and economical approach to single or multi-stage catalytic bed processing of a hydrocarbon.
A specific object of this invention is to provide improved radial flow reactor designs, and methods for using them, in connection with single or multi-stage catalytic bed processing of a hydrocarbon integrated with a thermal energy exchange system for either adding or withdrawing heat before, after, and/or between serial catalyst beds or adding and/or withdrawing heat upstream and/or downstream of a single catalyst bed.
Still another specific object of this invention is to provide an improved radial flow type reactor apparatus and methods for effecting single or multi-stage catalytic bed dehydrogenation of an alkylaromatic hydrocarbon to a corresponding alkenylaromatic hydrocarbon, specifically ethylbenzene to styrene.
Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the methods and related apparatus, involving the several steps and the various components, and the relation and order of one or more such steps and components with respect to each of the others, as exemplified by the following description and the accompanying drawings. Various modifications of and variations on the method and apparatus as herein described will be apparent to those skilled in the art, and all such modifications and variations are considered within the scope of the invention.
In the present invention, one or more annular-shaped catalyst beds are contained within the interior of a reactor shell, with heating or cooling being carried out in the core region of the reactor interior and/or in annular regions between serial catalyst beds or, alternatively, in front of and/or behind a single catalyst bed. In a representative embodiment, after leaving a first, inner catalyst bed, the process stream passes substantially radially through a reheat (or cooling) annulus containing heating (or cooling) means, such as one or more rows of heating (or cooling) tubes, followed by a mixing element (such as a set of perforated or slotted plates), before entering a second, outer annular catalyst bed. In a representative embodiment of the invention, a heat transfer medium flowing inside the heating (or cooling) tubes supplies heat to (or withdraws heat from) process gases. This scheme results in negligible reheat pressure drop and a substantial reduction in void volume as compared to the use of a more conventional external shell and tube heat exchanger. Consequently, process yield is improved and a significant reduction in equipment cost is achieved by elimination of two or more vessels and their associated piping.
In general, the heating or cooling means of this invention comprises a thermal heat exchange apparatus positioned relative to at least one annular catalyst bed such that gaseous process streams flowing radially into or out of any one or more annular catalyst beds are heated or cooled as desired. In one embodiment, the thermal heat exchange apparatus may be located in the core region of the reactor inside the annulus of a single annular catalyst bed or of the innermost catalyst bed of a series of radially-spaced concentric annular catalyst beds. In another embodiment, the thermal heat exchange apparatus may be located in the annular region surrounding the outside of a single annular catalyst bed. In another embodiment, a first thermal heat exchange apparatus may be located in the core region of the reactor and a second thermal heat exchange apparatus may be located in the annular region surrounding the outside of a single annular catalyst bed or in the annular region separating a first, inner annular catalyst bed from a second, outer annular catalyst bed. In similar fashion, additional radially-spaced concentric annular catalyst beds may be located within the reactor and additional thermal heat exchange apparatuses may be located between some or all of them, as well as in the annular region surrounding the outermost of those catalyst beds.